Process for coating flatlike surfaces

ABSTRACT

A process for coating a flatlike surface, such as the inner surface of the viewing panel of a cathode ray tube, comprising A. SLOWLY ROTATING THE SURFACE, FACING DOWNWARD, ABOUT AN AXIS THAT IS SUBSTANTIALLY NORMAL TO THE SURFACE, B. PROJECTING A STREAM OF LIQUID COATING MATERIAL IN A TRAJECTORY TO CONTACT THE ROTATING SURFACE SUBSTANTIALLY TANGENTIALLY AND THEN TO PASS ALONG THE SURFACE THROUGH THE AXIS, C. AND THEN PERMITTING EXCESS COATING MATERIAL TO DRAIN FROM THE ROTATING SURFACE.

United States Patent Smith Mar. 28, 1972 [54] PROCESS FOR COATING FLATLIKE 2,721,809 10/1955 Marks et al. ..117/104 x SURFACES 3,440,081 4/1969 Glovatsky et a1.

3,376,153 4/1968 Fiore ..117/97 x Bradford Knox Smith, Lititz, Pa.

[72] Inventor:

[73] Assignee: RCA Corporation [22] Filed: Dec. 22, 1969 [2]] App]. No.: 887,267

[52] U.S.Cl. ..1l7/97,117/33.5 C, 117/104R, 1l7/105.4,117/101,118/318 [51] Int. Cl ..B44d 1/08 [58] Field ofSearch ..l17/97,101,104R,l05.4,

[56] References Cited UNITED STATES PATENTS 2,792,810- 5/1957 Maconeghy et a1 ..118/55 X Hartley et al. ..118/55 X A process for coating a flatlike surface, such as the inner surface of the viewing panel of a cathode ray tube, comprising a. slowly rotating the surface, facing downward, about an axis that is substantially normal to the surface,

b. projecting a stream of liquid coating material in a trajectory to contact the rotating surface substantially tangentially and then to pass along the surface through the axis,

0. and then permitting excess coating material to drain from the rotating surface.

10 Claims, 3 Drawing Figures PHENTEumm I972 3. 652.323

' o INVENTOR.

5 EHADFURDKSMITH X/ W I BACKGROUND OF THE INVENTION This invention relates to a novel process for coating a flatlike surface of an article and particularly, though not necessarily exclusively, to a process for coating the inner surface of the glass viewing panel for a cathode ray tube.

One type of color television picture tube includes a light-absorbing matrix as part of the luminescent screen structure. This type of tube has been described previously; for example, in US. Pat. Nos. 2,842,697 to F. J. Bingley and 3,146,368 to J. P. Fiore et al. These patents describe color television picture tubes of the aperture-mask type (also called shadow-mask type) in which a light-absorbing matrix is located on the inner surface of the viewing window of the tube. In this structure, the matrix has a multiplicity of holes therein, each phosphor dot of the luminescent screen filling one hole in the matrix.

In one process for making the matrix, a pattern of polymeric dots is produced on the inner surface of the viewing window where the holes in the matrix are to be located. The surface and polymeric dots are then overcoated with a liquid suspension or slurry containing matrix material, such as graphite, and then dried. Then, the polymeric dots are removed taking with them the overlying matrix material but leaving the matrix material adhered to the inner surface around the areas previously occupied by the polymeric dots.

In this previous process, the step of overcoating the window and polymeric dots with a suspension containing matrix material was carried out in the factory by dispensing a quantity of the suspension upon the inner surface of a slowly rotating panel facing upward, rotating and tilting the panel to spread the suspension over the inner surface and then dumping or spinning off the excess suspension. While this procedure produces good coatings, the suspension has a tendency to flow over the edge, down the outer surfaces of the sidewalls and across the outer surface of the viewing window. The outer surfaces of the viewing window and of the sidewalls must then be cleaned, usually by hand, to remove this matrix material in order to produce a satisfactory product.

SUMMARY OF THE INVENTION The novel process comprises slowly rotating a downwardfacing, flatlike surface to be coated about an axis that is substantially normal to the surface, and preferably with the axis at about 30 from the vertical. A stream of liquid coating material is projected in a trajectory to contact the slowly rotating surface substantially tangentially and then to pass along the surface through the axis and then radially across the surface.

After at least one rotation, the stream is stopped and the excess coating material is permitted to drain from the rotating surface.

By projecting a stream of the suspension into substantially tangential contact with the rotating surface facing downward, several advantages are achieved. The suspension no longer tends to travel over the outer surfaces of the panel, thereby eliminating a manual cleaning step. The suspension travels at most over the edge and up the outer sidewalls only a short distance, which is easily removed by mechanical means. In addition, the coating step is completed more rapidly and the inner sidewalls are completely coated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional elevation of the viewing panel of a cathode ray tube being coated by the novel process at the preferred angle of about 30.

FIGS. 2 and 3 are sectional elevations of viewing panels for cathode ray tubes being coated by the novel process at angles of about 0 and respectively from vertical.

DESCRIPTION OF THE PREFERRED EMBODIMENT Example A preferred process is illustrated with respect to FIG. 1,

holder (not shown) slowly rotating about an axis 23. The panel 21 is generally bowl shaped comprising a viewing window 25 including a flatlike inner surface 27 and an outer surface 28. Sidewalls 29, having inner surfaces 31 and outer surfaces 33, rise from the edge of the viewing window 25 to define a generally rectangular configuration. The axis 23 is generally normal to and centrally located to the inner window surface 27 (which is to be coated) of the viewing window 25. The axis 23 is inclined from a vertical axis 35 by about 30. A pattern of polymeric dots adheres to the inner window surface 27.

The panel 21 is slowly rotated at about 6 to 10 rpm. about the axis 23 while a stream 37 of liquid coating material issues from an 0.25-inch (inside diameter) nozzle 39 under about 3.2 pounds-per-square-inch pressure. With this small pressure behind it, the stream is said to be limp and follows an arcing trajectory. The nozzle 39 is so located that the stream 37 is projected to contact the inner window surface 27 substantially tangentially above the axis 23, so that the coating material passes, after contact with the surface, through the axis and then radially across the inner surface 27. The liquid coating material is an aqueous suspension containing about 5 percent by weight of colloidal graphite. This suspension has a viscosity of less than 2.5 centipoises and a pH of about 10. Upon contact, the coating material follows the inner surface 27 because of interfacial tension and'then passesdown the inner sidewall surface 31 and drips off the edge 41 by gravity. A small amount of coating material passes over the edge 41 and up the outer sidewall surface 33, a distance less than about 0.50 inch.

The stream is continued while the panel 21 is rotated at least one revolution, preferably about 1.2 revolutions, to assure overlapping coverage. The stream 37 is stopped and rotation is continued while the excess coating material drains by gravity and the coating dries. The rotation may be increased at this point, for example, to about rpm. to aid in controlling the thickness of the coating. Also, drying may be accelerated by applying radiant heat to the coating. Coating material which is deposited upon the edge 41, the outer sidewall surface 33 and the stud 43, may be removed while the panel is rotating by applying thereto a jet of water with or without the aid of the mechanical action of a brush or sponge. After cleaning and drying are completed, the coated panel may be treated to remove the polymeric dots and overlying coating, while retaining the coating material around the areas previously occupied by the dots. The viewing panel 21 is then removed from its holder and the cycle may be repeated with another panel.

General Considerations The novel process may be applied to coating flatlike surfaces other than the inner surfaces of round or rectangular viewing panels for cathode ray tubes. The phrase flatlike surfaces is intended to include concave and convex surfaces with a shallow curvature. When applied to viewing panels for cathode ray tubes, the axis of rotation is best inclined at angles in the range of about 5 to 60 from the vertical. FIG. 2 shows the novel process being carried out on a viewing panel rotating about an axis 23a which is vertical; that is, the axis is said to be inclined 0 from vertical. In this position, the coating material does not deposit as far up the outer sidewall surface 33a, but the nozzle 39a is more difficult to position, and drainage from the central portion of the viewing window 25 is slower. FIG. 3 shows the novel process being carried out on a viewing panel which is rotating about an axis 23b which is inclined at about 60 from vertical. In this position, the nozzle 39b is more easily positioned, coating and draining of the coating material are more rapid, but the coating material deposits further up the outer sidewall surface 33b.

It can be seen that by facing the inner window surface 27 downward, the coating material drains by gravity tending to even out the coating and tending not to travel over the outer window surface 28. This is the opposite of the abovedescribed previous process in which a quantity of suspension is poured into an upward-facing inner window surface which is rotated and tilted to spread the coating material, and then the excess coating material is removed by dumping or by spinning off centrifugally. In the previous process, gravity and surface tension tend to leave parts of the inner sidewalls uncoated and tend to encourage some of the excess coating material to travel over the outer surface of the viewing window. These difficulties are avoided by the novel process which eliminates hand cleaning of the outer surfaces, produces an even and complete coating of the inner surfaces of the sidewalls and viewing window. The novel process also permits more rapid coating of the inner surfaces with simplified dispensing and salvage equipment.

The nozzle size and the pressure on the coating material at the nozzle are not critical except that a limp stream should be produced so that, at the point of contact of the stream with the surface to be coated, substantially tangential contact is achieved. With this tangential contact, substantially no erosion or scouring of the structure or coatings, such as polymeric dots, on the inner surfaces occurs. Also, the coating material spreads by its own momentum guided in part by its own wetting characteristics. High velocities and angles substantially greater than tangential cause the liquid coating material to bounce off the inner surface leaving bare spots and'requiring equipment to recover the splash or spray produced. Spraying the coating material (instead of limp stream application) produces the same difficulties, though to a somewhat different degree. Pressures of 5 pounds per square inch and less are preferred. In the novel process, a solid stream of liquid coating material is applied with substantially no spray or splash produced.

The liquid coating material'may be of any composition. In the example, the coating suspension preferably contains 4.5 to 6.5 percent by weight of colloidal graphite. Part or all of the graphite in the example may be replaced with other particulate materials, with or without a binder. Some other light-attenuating materials that may be used for making a light-absorbing matrix are other forms of carbon such as carbon black and acetylene black, and other pigments, such as ferrous oxide and manganese dioxide. Broadly speaking, the novel process may be used for depositing any pigmented or unpigmented liquid coating composition of relatively low viscosity upon a flatlike surface.

The liquid coating composition must have a sufficiently low viscosity to be projected as a stream and then to flow across the surface to be coated. The lower the viscosity, the faster the surface can be covered and drained of excess coating material. In the case of suspensions of carbon particles in a liquid, viscosities of about 1 to 50 centipoises can be used, preferably about 5 centipoises and lower. The lower the viscosity, the faster may be the speed of rotation of the surface to be coated. The speed of rotation should be such that wetting of thesurface by the coating material advances uniformly without missing any surface areas.

It is preferred that the surface be rotated at least about 1.2 revolutions during application to assure complete coating of the surface. Low rotational speeds assure that good coating can be obtained without producing voids and without using an sists essentially of particles of light-attenuating material in a excessive volume of coating material. Small voids are difi'icult to cover in a subsequent pass due to the presence of an established wetted path. It is also preferred that the stream be provided in a smooth flow of liquid coating material. This may be achieved conveniently with pressures produced from a head tank.

After the liquid coating material has covered the surface, the stream is stopped but the rotation of the surface is continued. Rotation aids in smoothing the applied material quickly. If desired, the rotational speed may be increased, up to about rpm. As the rotational speed is increased, centrifugal forces become greater and accelerate the draining and smoothing of the applied material, especially near the edge of the surface. However, centrifugal forces do tend to drive excess coating material further up the outside surfaces of the sidewalls.

I claim:

1. A process for coating 5 flatlike surface comprising a. slowly rotatin g said surface facing downward about a axis that is susbstantially normal to the surface,

b. projecting a stream of liquid coating material in a trajectory to contact said rotating surface substantially tangentially and then to pass along said surface through said axis,

c. and then permitting excess coating material to drain from said rotating surface. i

2. The process defined in claim 1, wherein said coating material has a viscosity of 50 centipoises or less.

3. The process defined in claim 2 wherein said liquid stream is projected at low velocity under pressures of 5 pounds per square inch and less.

4. The process defined in claim 2 wherein said axis is inclined at an angle of 50 to 60 from vertical.

5. A process for coating the inner flatlike surface of the viewing panel of a cathode ray tube comprising a. supporting the panel with said inner surface facing downward,

b. slowly rotating said panel about an axis that is substantially normal to said inner surface,

c. projecting a limp stream of liquid coating material in an arcing trajectory into substantially tangent contact with said rotating inner surface in a path to pass said contacted material through said axis and then radially across said surface,

d. and then permitting excess coating material to drain from said rotating surface.

6. The process defined in claim 5 wherein said liquid coating material has a viscosity of 50 centipoises or less and convolatile liquid.

7. The process defined in claim 6 wherein said liquid coating material consists essentially of carbon particles in water.

8. The process defined in claim 7 wherein said axis is inclined at an angle of 50 to 60 from vertical.

9. The process defined in claim 7 wherein said axis is inclined at an angle of about 30 from vertical.

10. The process defined in claim 7 wherein said liquid coating material consists essentially of colloidal graphite particles in water.

i i l 

2. The process defined in claim 1 wherein said coating material has a viscosity of 50 centipoises or less.
 3. The process defined in claim 2 wherein said liquid stream is projected at low velocity under pressures of 5 pounds per square inch and less.
 4. The process defined in claim 2 wherein said axis is inclined at an angle of 50* to 60* from vertical.
 5. A process for coating the inner flatlike surface of the viewing panel of a cathode ray tube comprising a. supporting the panel with said inner surface facing downward, b. slowly rotating said panel about an axis that is substantially normal to said inner surface, c. projecting a limp stream of liquid coating material in an arcing trajectory into substantially tangent contact with said rotating inner surface in a path to pass said contacted material through said axis and then radially across said surface, d. and then permitting excess coating material to drain from said rotating surface.
 6. The process defined in claim 5 wherein said liquid coating material has a viscosity of 50 centipoises or less and consists essentially of particles of light-attenuating material in a volatile liquid.
 7. The process defined in claim 6 wherein said liquid coating material consists essentially of carbon particles in water.
 8. The process defined in claim 7 wherein said axis is inclined at an angle of 50* to 60* from vertical.
 9. The process defined in claim 7 wherein said axis is inclined at an angle of about 30* from vertical.
 10. The process defined in claim 7 wherein said liquid coating material consists essentially of colloidal graphite particles in water. 